Persistence of limited-transpiration-rate trait in sorghum at high temperature

• Sorghum is major crop in hot, dry regions of the world.
• Limited transpiration rate under high vapor pressure deficit is an asset.
• Genotypic variation for transpiration response at 37 °C.
• Opportunities to select sorghum genotypes to fit various climatic conditions.

A limited-transpiration (TRlim) trait has been identified in many crop species, including sorghum (Sorghum bicolor (L.) Moench), that results in restricted transpiration rate under high vapor pressure deficits (VPD). The benefit of TRlim is that under high midday-VPD conditions crop water loss is limited so that there is water conservation and positions the crop to better withstand later-season drought. Previous studies performed at 31 °C found that TRlim was commonly expressed among sorghum genotypes. It was also found that those lines with low VPD breakpoints for expression of the TRlim trait exhibited insensitivity in transpiration rate to being fed silver ions. However, it is uncertain how applicable these previous results obtained at 31 °C might be at higher temperature that may exist at midday in regions where sorghum is commonly grown. The current study tested for the expression of TRlim at 37 °C in 16 sorghum genotypes previously found to express the trait at the lower temperature. Only three of the genotypes sustained expression of TRlim at 37 °C. These results indicate that for environments where temperature may commonly reach or exceed 37 °C, sorghum genotypes have been favored that acclimate to the high temperature by losing the TRlim trait. In conditions in which very high temperatures threaten crop heat stress, those genotypes that lose the TRlim trait at high temperature may be more desirable since increasing transpiration rates at these temperatures can result in leaf cooling. The silver test failed to discriminate genotypes for expression of the TRlim trait at high temperature.